1. Technical Field
The invention relates generally to a semiconductor device and a manufacturing method thereof. More particularly, the invention relates to a semiconductor light-emitting device and a manufacturing method thereof.
2. Related Art
With the progress in optoelectronic technologies, the manufacturing and the applications of a light-emitting diode (LED) has gradually matured. Due to the advantages of having low pollutants, low power consumption, and long lifetime, etc., the LED has been extensively applied as a light source or for illumination, such as in traffic signals, outdoor bulletin boards, and in the backlight source of displays. The LED has therefore become an important optoelectronic industry.
Nowadays, group III-V nitride materials are used in the LED. A main reason for adopting group III-V nitride materials is that most have direct bandgap structures with emitting wavelengths ranging from the infrared (IR) to the ultraviolet (UV). A required emitting wavelength can be obtained by properly adjusting the constituent proportions of the group III-V nitride materials.
However, since the positive and negative charge centers in a unit cell of the group III-V nitride materials do not overlap, a dipole moment forms during the crystal growth process. A polarization phenomenon occurs naturally without an externally applied electric field, which is also referred to as the spontaneous polarization effect. Moreover, during the crystal growth process, because of the lattice mismatch between the epitaxial layer and the substrate, electric charges between different layers accumulate at the interface due to the force generated by the lattice deformation, which is also referred to as the piezoelectric polarization effect.
The existence of the aforementioned polarization effects (e.g., including the spontaneous polarization effect and the piezoelectric polarization effect) will affect the energy band structures of the semiconductor light-emitting device. These effects are especially pronounced in the quantum wells (QWs), also referred to as the quantum-confined Stark effect (QCSE). In this situation, the energy band will severely slanted under the influence of the strong polarized electric fields, such that the electrons and holes are respectively confined at the two sides of the quantum wells. Accordingly, the degree of spatial overlap of the electron and hole wavefunctions is reduced, the radiative lifetime is increased, and the radiative recombination rate of the electrons and holes and the internal quantum efficiency (IQE) are reduced, and thus lowering the light-emitting efficiency of the semiconductor light-emitting device. Therefore, researchers are urgently working towards enhancing the light-emitting efficiency of the semiconductor light-emitting device by reducing the impact that polarization effects have on the semiconductor light-emitting device.